The invention relates to an actuator for a pneumatic brake system and a method for making such an actuator.
Vehicles, particularly commercial vehicles such as trucks and buses, typically have pneumatic braking systems. These pneumatic brake systems typically include an air-operated actuator for applying a disc brake or a drum brake at a wheel of the vehicle. A typical pneumatic actuator, referred to as a “spring brake actuator,” includes a power spring which biases the actuator in a “brake applied” position when a parking brake function is desired or an emergency brake activation is needed when the service brake function is disabled. The power spring is compressed by compressed air in parking brake chamber to release the brake for normal service brake operation, with the spring being arranged to apply the brake when the compressed air is released in parking or emergency situations.
As illustrated in prior art FIG. 1, a typical pneumatic actuator 1 includes a housing 2 (also referred to as the cylinder), a spring pressure plate 3 (also referred to as the parking brake piston) moveably disposed in the housing, with a diaphragm 4 that is sealingly attached to the housing 2 and the spring pressure plate 3 to seal between the housing 2 and the spring pressure plate 3. The housing, spring pressure plate and diaphragm define a parking brake air chamber 5 for receiving compressed air to force the spring pressure plate 3 to compress the power spring 6, thereby releasing the parking brake. When the power spring 6 is compressed its actuator rod 7 no longer biases the service brake piston 8 and its actuator rod 9 toward the brake application position, thereby freeing the service brake piston 8 to be used to provide the service brake function of the spring brake actuator 1.
Typically, an outer flange 10 of the diaphragm 4 is captured between opposing flanges of the spring brake actuator housing, with the opposing flanges being held tightly together with a clamping ring 11 (for example, a separate clamping ring or a crimped-over flange of one portion of the housing). In the FIG. 1 illustration, the opposing flanges of the housing include the flange 12 of adapter base 13 and the flange 14 of the non-pressure plate (NPP) portion 15 of the housing 2.
FIG. 2 is a detailed partial cross-section view of the diaphragm capture arrangements of a known spring brake actuator, showing an outer flange 10 of diaphragm 4 having a wedge-shaped portion 16 captured between the housing opposing flanges 12, 14. In this embodiment, the opposing flanges are held together with a separate clamping ring 11.
A concern with such known diaphragm arrangements is leakage across the diaphragm's outer flange, for example, due to uneven compression from a clamp band used to hold flanges 12, 14 together, particularly at the clamp band's clamping bolt location. A further concern is that in some known spring brake actuators the wedge-shaped outer flange of the diaphragm has small sealing points in the region between the opposing flanges, which can cause sealing problems, as well as making difficult installation of a replacement diaphragm on a vehicle.
An objective of the present invention is to provide a diaphragm arrangement which addresses these and other problems by providing an improved seal having increased resistance to leakage across the sealing surface. The improved diaphragm seal arrangement is particularly well suited for use with parking brake actuators which have bolt-on NPP actuator housing portions.
The diaphragm of the present invention is a cup shaped diaphragm which includes two offset sections along its outer perimeter. The first offset section extends generally perpendicular to the central axis of the diaphragm. The second offset section extends generally parallel to the central axis of the diaphragm. The diaphragm may also include a wedge shape with a domed outer portion along the outer perimeter of the diaphragm.
These diaphragm features are configured to cooperate with matching annular spaces between the adapter base and the non-pressure plate of the actuator. The offset sections configuration improves the centering of the diaphragm in the actuator during the installation process (during both initial assembly and subsequent repair operations). The wedge portion may be provided to further enhance the diaphragm's resistance to being pulled out of space between the opposing flanges during in-service use. In one embodiment, the annular space between the actuator housing portions includes round surfaces to lower local stresses in the diaphragm material.
An embodiment uses at least two bolts along the external periphery of the brake actuator housing to hold the adapter base to the NPP housing portions together, eliminating a need for a clamping band. The entire annular area between the non-pressure plate and adapter base is filled by the diaphragm offset portions for an improved seal and all sealing takes place at this portion of the diaphragm once the bolts are installed, thereby eliminating any need to rely on the wedge portion of the outer circumference of the diaphragm to provide sealing or to ensure sealing is provided in the outer peripheral region around the housing closure bolts.
A further benefit of the present invention is that actuator replacement may be performed without the need to remove the NPP portion of the actuator from the vehicle.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.